This invention relates to a vehicle antilock control system and method and specifically to a system and method of determining a vehicle reference speed in an antilock brake control system.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of wheel slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is reached. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value. An antilock control system achieves this objective by detecting an incipient wheel lockup condition and releases pressure at the wheel brake to allow recovery from the incipient wheel lockup condition. Criteria typically used to indicate an incipient wheel lockup condition includes excessive wheel slip.
Since wheel slip is the relative difference between vehicle speed and wheel speed, any inaccuracies in the determination of the reference speed representing the vehicle speed will result in inaccuracies in the computed wheel slip.
A method for establishing the vehicle speed reference based on wheel speed information provides for allowing one of the wheels of the vehicle during antilock controlled braking to free-wheel so as to assume a speed equal to the vehicle speed. This is accomplished by maintaining lower braking forces on the selected wheel to allow the wheel to accelerate to the vehicle speed. This speed then provides an accurate estimate of the vehicle speed and may be utilized to accurately compute the wheel slip values upon which antilock control braking is based. This invention is directed to this form of determining an accurate vehicle reference speed during antilock controlled braking.